1. Field of the Invention
The present invention relates to a control system for a plant in which a plurality of controlled variables are controlled by a plurality of control inputs, respectively, and interaction occurs between the control inputs and the controlled variables.
2. Description of the Related Art
Conventionally, as a control system of this kind, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2006-132429 is known. An internal combustion engine to which is applied this control system is configured such that the lift of intake valves can be changed. Further, in the conventional control system, a pressure within an intake passage (hereinafter referred to as the “intake pressure”) is controlled by controlling the degree of opening of a throttle valve disposed in the intake passage to a target opening thereof, and the amount of intake air drawn into the engine (hereinafter referred to as the “intake air amount”) is controlled by controlling the lift of the intake valves to a target lift thereof. In this case, not only the intake pressure but also the intake air amount is changed by controlling the degree of opening of the throttle valve, and further not only the intake air amount but also the intake pressure is changed by controlling the lift of the intake valves. As is apparent from this, interaction occurs between the target opening and the target lift as control inputs, and between the intake pressure and the intake air amount as controlled variables.
To avoid this inconvenience, the conventional control system includes a non-interacting controller for suppressing the above-mentioned interaction. Further, in the conventional control system, based on a non-interacting control law (cross-controller), a system formed by combining the non-interacting controller and the engine is modeled as a linear imaginary plant with no interaction. Furthermore, based on a model equation obtained thereby, the target opening of the throttle valve and the target lift of the intake valves are calculated as non-interacting inputs capable of suppressing the interaction.
As described above, in the conventional control system, the system formed by combining the non-interacting controller and the engine is modeled as the linear imaginary plant with no interaction based on the non-interacting control law, but since the engine is a non-linear system, it is difficult to model the engine by the above-mentioned method. Further, in a case where the conventional control system is applied to a plant with stronger non-linearity than that of the engine, the plant cannot be modeled, which causes a fear that the interaction cannot be suppressed. Further, if control inputs and controlled variables are different, the manners and degrees of interaction are also different, and hence modeling equations as well are different. Therefore, it is necessary to set modeling equations for a plurality of control inputs and a plurality of controlled variables which are different from each other in the manner of interaction and the like, from the beginning, based on the non-interacting control law. This makes the design of the control system very complicated. In view of the above-mentioned point, there is room for improvement of the conventional control system.